State-of-the-art brushless d.c. motors may utilize rare earth, permanent magnets in the rotors thereof because of the inherent resistance of such magnets to demagnetization by the electromagnetic fields resulting from the motor's energization. Accordingly, these motors are capable of achieving high output torques with increased field energization, yet with relatively low attendant risk of rotor demagnetization. Moreover, rare earth magnets render such motors more compact, lighter, and more efficient than prior art motors employing conventional permanent magnets. However, because of the relatively high intensity magnetic fields associated with rare earth permanent magnet motors, the output of such motors is often limited by the temperature rise in the stator coils due to the high electrical energization thereof.
The electromechnical actuation system for a cruise missile fin is an example of an application for which permanent magnet brushless d.c. motors are well-suited. When the missile is fired, high motor torque is required for initial fin deployment. As the missile cruises, a low energy input to the motor is sufficient to maintain the fin in position. However, as the missile approaches a target and adjustment of fin position is required for final aiming, high motor torque output is required to hold the fin steady against aerodynamic forces. Such high torque outputs are, of course, only achieved with a high electrical input to the motor, thereby significantly raising the temperature of the stator coils. Since the motor operates in an extremely close environment and at relatively low speeds, cooling of the motor by forced ventilation techniques is generally unfeasible. Furthermore, since the coils, and in particular, the ends thereof, are usually spaced from the stator core by an air gap, coil cooling by thermal conduction to the stator core is generally ineffective. Accordingly, improved methods for cooling brushless d.c. motors are currently sought.